Wireless network systems, for example, wireless communication systems or wireless sensor systems, where a plurality of transmission units send data to one or more receiver units by means of wireless signals, are known in the prior art.
They can be classified in various ways, including, based on the nature of the wireless signals (optical, acoustic, electromagnetic, etc.). For the present invention only RF signals are relevant. There are many different modulation techniques, each with their own specific advantages and disadvantages. For example, amplitude modulation (AM) is highly suited for sending low-frequency analogue signals (for example, speech or sound), and QPSK is highly suited for sending/receiving digital satellite signals, and QAM is highly suited for sending/receiving digital signals over a coaxial cable, etc., but there are plenty of other modulation techniques.
The frequency band in which is communicated is also important. It is known that the electromagnetic spectrum is strongly regulated and the frequency spectrum is divided into different frequency bands, which are assigned to specific applications. For the present invention, the ISM bands are particularly relevant, and particularly the so-called 433 MHz band, the 784 MHz band in China, the 868 MHz band in Europe, and the 915 MHz band in the US and Australia.
Different standards make use of communication in the ISM band, for example, WiFi, ZigBee and Bluetooth, and all use some form of frequency spread (spread spectrum). For example, WiFi and Bluetooth use ‘Frequency Hopping Spread Spectrum’ while ZigBee uses ‘Direct Spread Spectrum’. The bandwidth of channels in Bluetooth is about 1 MHz, and ZigBee-based networks use a bandwidth of 0.3 MHz, 0.6 MHz or 2 MHz, depending on the frequency where in which these networks communicate.
Radio network configurations can be classified into two broad categories: (a) so-called ‘star networks’, wherein each transmission unit communicates directly with one single receiver unit, and (b) ‘mesh networks’, wherein some of the transmission units also act as receivers, and can pass on signals. Such units are also referred to as ‘signal amplifiers’ (or repeaters). Mesh networks are extremely popular with wireless sensor networks. However, the protocol stack of such systems can be very complicated. The most well-known protocol stack is probably ‘ZigBee’.
Wireless network systems can be further classified depending on their power supply. For example, ZigBee and Bluetooth are specially designed for low power consumption, which makes these technologies useful for applications with battery power supply.
US20100054307 describes a system for the monitoring of events. The system includes a sensor, a wireless transceiver connected to the sensor, and a base unit with a transceiver that selectively communicates with the sensor units. The sensor units periodically send out a message to the base unit, which is confirmed by the base unit.
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